1. Field of the Invention
The present invention relates to a method for vaporizing an anesthetic liquid of the type wherein a carrier gas is passed through a container, which is at least partially filled with anesthetic liquid with the carrier gas saturated with vaporized anesthetic being supplied to an anesthetic apparatus for mixture with other gas components to form a breathing gas.
The present invention also relates to a vaporizer of the type operating as generally described above.
2. Description of the Prior Art
The most common way of supplying anesthetic to a patient who is to be anesthetized is to vaporize a specific amount of an anesthetic liquid in a vaporizer, disposed at an anesthetic apparatus, and to deliver this anesthetic in a gas mixture consisting of oxygen and nitrous oxide (and sometimes air) to the patient via the apparatus. Many types of vaporizers are available. The various vaporizers differ in particular in the way in which anesthetic liquid is vaporized and conveyed with the carrier gas. The vaporizer can be located on the low-pressure side of the anesthesia system, in which gas pressure is the pressure to which the patient is subjected, i.e., about atmospheric pressure, or on the high pressure side, in which gas pressure exceeds normal atmospheric pressure, i.e., usually 2 to 6 bars of gauge pressure.
One type of high-pressure vaporizer is described in the brochure "Halothance Vaporizer 950/Enflurane Vaporizer 951/Isoflurane Vaporizer 952"--Operating Manual, AG 0188 7, Siemens-Elema AB, 1988. The anesthetic liquid is in a container on the highpressure side of an anesthetic system. One line carries a relatively large flow of a gas mixture through the vaporizer and on to the anesthetic apparatus. A throttle valve is disposed within the vaporizer and upstream from the throttle valve there is an opening between the line and the container. Downstream from the throttle valve a nozzle is located in the line. The nozzle is connected to a tube immersed in the anesthetic liquid. A pressure drop is caused across the throttle valve due to the flow of the gas mixture through it. As a result of the pressure gradient, anesthetic liquid is pressed out through the nozzle into the lire and is vaporized by the passing flow of breathing gas. The amount of anesthetic liquid to be vaporized is regulated by changing the position of the throttle valve and, accordingly, the magnitude of the drop in pressure. When no anesthetic liquid is to be vaporized, a valve is reset, and the gas mixture bypasses the vaporizer. Since the pressure drop across the throttle valve is also governed by absolute pressure, the vaporizer must be calibrated for an entering pressure, or range of pressures, which may be a source of error. Another possible source of error which must be taken into account with this type of vaporizer is the container's compressible volume and the amount of anesthetic liquid contained in the container.
One type of low-pressure vaporizer is described in European Application 166 305. In this vaporizer, a specific amount of anesthetic liquid is delivered to a hot plate in a vaporizing chamber. A total flow of a gas mixture passes through the vaporizer and conveys the vaporized anesthetic liquid. The amount of liquid sent to the vaporizing chamber is therefore governed by the magnitude of the gas flow and the desired concentration of anesthetic in the gas mixture. Since two factors affect the course of events, the risk of erroneous dosing is increased. Even if such errors are usually not large enough to pose any risk to the patient, they do exist. Moreover, this type of vaporizer is hard to replace while in operation. One version of this type of vaporizer is described in PCT Application WO 92/19303.
Second and third types of low-pressure vaporizers are described in U.S. Pat. No. 3,794,027. The second type of low-pressure vaporizer has a container holding anesthetic liquid. A wick is partially immersed in the liquid. The gas can be carried, via a manually switchable valve, either through or past the low-pressure vaporizer. When gas is sent through the low-pressure vaporizer, it passes the wick which has drawn up anesthetic liquid. Liquid in the wick is vaporized by the passing flow of gas and is carried, with the gas, to the patient. In principle, vaporization is controlled by the physician switching the valve so gas is channeled through the low-pressure vaporizer for specific periods of time. The system's efficacy is therefore completely dependent on the physician's skill and experience.
The third type of low-pressure vaporizer also has a container filled with anesthetic liquid. A flow of gas at a relatively low pressure is connected thereto. The flow of gas is then allowed to bubble through the anesthetic liquid so as to become saturated with vaporized anesthetic liquid. The gas is then carried with anesthetic gas to the anesthetic machine and the patient. As is the case with the second type of low-pressure vaporizer, this type utilizes manual bypass for specific periods of time in regulating the end concentration of anesthetic.
Thus, a problem encountered with most types of anesthetic vaporizers is a lack of accurate control of the amount of anesthetic liquid vaporized. Since the gases are involved, both compression and flow affect the final concentration of anesthetic gas in the breathing gas. As the above shows, different techniques are used in attempts to control the amount of anesthetic liquid to be vaporized. The last vaporizer mentioned, in which breathing gas is saturated with vaporized anesthetic, has the advantage that the anesthetic liquid's partial pressure is well-defined at a given temperature and pressure. This type of vaporizer, however, is riot without problems. Since it is directly connected to the patient circuit in some way, it exhibits a large compressible volume, making control of the amount of anesthetic in the system more difficult. Switching from one anesthetic to another is a complex operation, and there is always a risk of leakage through backflow, and saturation of breathing gas passed through the vaporizer must be assured.
Instead of passing the entire flow of gas through the low-pressure vaporizer, for specific periods of time, to be saturated with anesthetic, a small, exact, partial flow of gas can instead be passed, almost continuously, through the low-pressure vaporizer in order to be saturated with anesthetic. One such design is described in European Application 496 336.